As important techniques for improving structural safety, disaster prevention, disaster mitigation, and intelligent management and maintenance in large-scale civil work and traffic engineering, structural health monitoring techniques have received extensive attention. Since the 1990s, the techniques have been studied comprehensively and applied in engineering projects in USA, Europe, and Asia (including Japan, China and Korea). Presently, structural health monitoring systems at different levels have been installed for large-scale new bridge structures in, for example, China.
However, most key sensing techniques in existing structural health monitoring systems for large-scale engineering structures in civil work and transportation infrastructures are sensing techniques that were applied early in aeronautic and astronautic, military, and precision machinery engineering, and even most structural damage identification theories were developed for homogeneous and small-size structures rather than large-scale civil work and transportation engineering structures. Conventional sensors have poor durability and are subject to severe interferences in sensing and data transmission. Therefore, they are only applicable to short-term and small-scale detection, but are generally unsuitable for long-term real-time monitoring in an embedded state, and cannot meet the requirement for long-term detection and structural health monitoring.
At present, the sensing and monitoring techniques for large-scale engineering structures such as bridges and tunnels are mainly classified into two types. One is a local sensing and monitoring technique, represented by conventional strain foils. The local sensing and monitoring technique is too “local” to capture the damage.
The second is a global sensing and monitoring technique, represented by accelerometers. For global sensing and monitoring techniques, it is so global that the measurements are in a quite poor relation to the damage. Furthermore, it is difficult if not impossible to employ the conventional sensing and monitoring techniques with a high cost and short sensing gauge for large-scale distributed deployment for large-scale civil work and transportation engineering structures. For example, for minor damages, if the strain foils or accelerometers are deployed away from the damage, it is difficult for such sensors to monitor damage information. However, if the strain foils or the like are deployed at some damages, such as cracks, the sensors may be damaged easily. Moreover, for dynamic measurement, it is difficult for an identification method based upon the global sensor dynamic measurement to reflect the overall performance of the structure and effectively capture the unpredictable damages of the structure.
With respect to the technical bottleneck of the application of the local sensing technique represented by conventional strain foils and the global sensing technique represented by accelerometers in the large-scale civil work and transportation engineering, it is necessary to propose new concepts and develop new sensing techniques on an area and distributed sensing for the area monitoring of critical zones of large-scale civil work and transportation engineering structures. Prof Zhishen Wu, Southeast University, P. R. China, and his team carried out technical research for improving the gauge length of traditional FBG sensors and developing long-gauge sensors. Chinese patent No. ZL 200610097290.1 discloses a distributed long gauge fiber Bragg grating (FBG) sensors, which is packaged by fiber-reinforced polymer (FRP) composites to make the traditional “point” FBG sensors have a distributed sensory function with a long gauge length, so as to achieve sensitizing enhancement and temperature self-compensation. However, the stability for long-term sensing and monitoring and the durability of the sensors need to be further improved. That Chinese patent does not consider the slippage problem of both anchoring ends of the long gauge sensors either. For large-scale engineering, such as civil work and transportation engineering, sensors used for long-term detecting and structural health monitoring need to have good stability for long-term sensing and durability, but the present conventional sensors can hardly meet the performance requirement of long-term monitoring in civil work and transportation engineering fields.